Although the cellular signal transduction pathways involved in growth control of cells are not fully understood, it is known that there are at least four types of proteins that participate in the process: growth factors, receptors, intracellular signal transducers, and intranuclear transcriptional controllers. Growth factors are one way a cell sends a message to another cell. The cell responding to the growth factor has a specific receptor that is sensitive to the growth factor. Since different cells have different receptors, each signal can produce a response in some cell types but not in others. Occupancy of growth factor is sensed by the cell and if metabolic conditions are favorable, the cell completes the growth cycle and gives rise to two cells similar to the original.
The chain of events that leads to cell growth is initiated by growth factor-receptor interactions which produce a signal in the cell. The cell interprets the signal and often stimulates intracellular signal transducers, or secondary messengers, that alter transcription, either by expressing new genes or modifying levels of expression of presently active genes. Not all second messenger activity is focused on transcription. Changes in cell shape and metabolism, for example, can be induced by the action of signals directly on existing cellular proteins.
The transcriptional response to a growth signal alters the protein composition of the cell providing the critical proteins needed for cell growth. Transcription is controlled by two types of DNA sequences: promoters, located at the start of transcription, and enhancers, located farther from the start site and able to act over long distances. Both types of sequences function through specific intranuclear transcriptional controllers or binding proteins that recognize short sequence motifs within the promoters and enhancers. These bound factors then accelerate or retard the initiation of transcription of proteins for cell growth.
Control of cell growth is an important aspect of an animal's physiology. Cells of an adult must divide frequently enough to allow tissues to remain in a steady state and division must be stimulated in instances where an immune response is required or where cell division must be stimulated, for example, at the site of a wound. Uninhibited growth of cells may result in tumor formation, and therefore control of cell growth is an area of considerable interest, particularly as it relates to cancer research. Therefore, it would be beneficial to understand cellular signal transduction pathways because of the potential for treating proliferative disorders.